A very thorough review of the effects of gingko upon the body, and when taken in conjunction with other medicines, including warfarin and aspirin, can be found at the American Academy of Family Physicians website at http://www.aafp.org/afp/20030901/923.html.
This article gives a round-up, albeit not very thorough, of the 50 or so studies to date on gingko, and further ongoing clinical trials in progress, which I am sure Hopkins' specialists will be watching closely and reporting on in due course.
In the same way that not everyone can take aspirin safely, not everyone can take gingko safely, and as will all herbal supplements, there is very little quality or dosage control as to what is claimed on the label, versus what is in the pill itself, versus what you are putting in your body.
The 'warning' nature of the message, I am sure, is also for people who never think to mention vitamins and supplements they are taking, or assume because it is a 'natural' remedy, that it is completely harmless. As this article points out, Vitamin K and even green leafy vegetables can be potentially dangerous! if not regulated in conjunction with warfarin intake, which need to be monitored on a regular basis.
As always, the take home message here is to stay as informed as possible, and work as closely with your doctor as possible.
Many thanks for another very useful article!